The subject of the present invention is a device for controlling the trim of a vehicle with four wheels, each equipped with a damper and with a suspension spring.
It is known that the operation and characteristics of the vehicle suspension are governed by three key parameters: the trim, the stiffness, provided in mechanical systems by a spring, and the damping, provided by a hydraulic damper or a friction device.
In order to stop the vehicle from rolling, especially during cornering, most vehicles are currently equipped with anti-roll bars of fixed stiffness, with one bar per axle, this bar being connected to the springs of the suspension. However, in some cases, these bars encounter siting difficulties, and their overall size may be a problem. What is more, as soon as the loads are not strictly symmetrical on the two wheels of one axle, the stiffness of the anti-roll bar stiffens the suspension, which constitutes another significant drawback of this type of embodiment.
More recently, three types of suspension have been proposed, aiming in general to improve the road holding of the vehicle and the comfort of the occupants. Thus, a first type of so-called "active" suspension is contrived so that it anticipates the dynamics of the connection of the vehicle with the ground with regard to the essentially random layout of the road, incorporating the parameters relating to speed, turn radius, load, acceleration/deceleration, ride height. For this purpose the suspension includes a servo valve associated with a hydraulic actuator and driven by a control solenoid valve. However, this suspension does not have a spring and therefore does not fulfill the stiffening function. What is more, it exhibits the drawback of requiring very significant amounts of hydraulic and electrical energy in order to fulfill the stiffening function despite the absence of the spring, so as to allow the stored-up energy to be restored. In effect, the electrical energy required may be of the order of 20 to 30 Kw.
The object of the so-called "semi-active" suspension, while retaining the existing elements and especially the dampers, is to vary the reference point at which the suspension elements bear, by incorporating all or some of the aforementioned parameters. For competition vehicles, the prime objective is therefore to maintain the ground effect as far as possible. In this "semi-active" suspension, the actuator is coupled to a hydropneumatic damper, which fulfills the stiffening function by the compression of the gas. The damping function may be fulfilled by a restriction between the actuator and the hydropneumatic cylinder, and allows the ground clearance to be corrected.
This device, used in some top-of-the-range touring vehicles, makes it possible to manage the rolling and pitching in ways other than with bars of fixed stiffness. Depending on the required dynamics, it is then possible to assume that the energy required is brought down to approximately 1/5 to 1/10 of the energy necessary for an active suspension.
Nevertheless, a "semi-active" suspension does not allow dynamic correction of the trim, because it is only possible to adjust it for two or three predetermined ground clearance values. Furthermore, its time constants are high.
Finally, a third type of embodiment recently mounted on top-of-the-range touring vehicles, is so-called "variable" suspension, which includes dampers with a programed damping law, instead of the conventional dampers. Such a device is passive, in contrast to the two aforementioned types of suspension, because the only external energy required is that which causes the orifice plates of the damper to vary, upon a command from a suitably programed computer (it being possible for this suspension to be associated with anti-roll bars).
However, the variable suspension does not incorporate the stiffness function, because the computer only drives the damping, that is to say the flow rate of fluid through a restriction. Thus, this suspension too does not make it possible to perform dynamic management of the trim of the vehicle, that is to say in fact to monitor the position of the whole of the vehicle with respect to the ground (ride height for each axle, angle of roll, pitch, ...) in each of the various situations which follow on from each other during a road journey.